Glycoalkaloids refer to a group of plant-derived compounds, which are also called steroidal alkaloids. The glycoalkaloid structure is composed of C27 isoprenoids containing a nitrogen atom, and it has been reported that there are 422 compounds of glycoalkaloids from plants belonging to the genus Solanum (Non Patent Literature 1, chapter 7.8). As to a plant belonging to the family Solanaceae other than those belonging to the genus Solanum, some plants belonging to the family Liliaceae are also known to contain glycoalkaloids. Among glycoalkaloids, important ones are chaconine and solanine from potatoes (Solanum tuberosum), and tomatine from tomatoes (Solanum lycopersicum), which belong to the genus Solanum of the family Solanaceae.
The potato is the world's forth largest crop produced, following maize, rice, and wheat. However, it is a well-known fact that buds sprouting from the tubers and the aerial parts of the plant contain chaconine and solanine, which are toxic substances. Chaconine and solanine cause poisoning symptoms such as abdominal pain, vertigo, and mild disturbance of consciousness. Also, chaconine and solanine easily accumulate in the tubers as a result of damage or exposure to sunlight; therefore, there is a risk that faulty handling of tubers may lead to an episode of accidental poisoning.
Accidental poisoning is occasionally observed, and in a recent case, there was an episode of accidental glycoalkaloid poisoning in an elementary school in Nara-city, Japan, on Jul. 16, 2009 (reported by Asahi.com). Potatoes are normally safe foodstuffs as the tubers are handled so that the glycoalkaloid levels are kept at 20 mg/100 g or less by, for example, dark storage of the tubers. However, in consideration of the risk of accidental poisoning such as the aforementioned event, reduction of glycoalkaloids in potatoes is a matter of concern to anybody involved in the handling of potatoes such as breeding, production, storage, transportation, sales, and purchasing of potatoes. Nevertheless, reduction of glycoalkaloids in potatoes has not been successful up to now. The reasons for this are that there is no wild potato species free from glycoalkaloid, and also, because the biosynthetic pathway of glycoalkaloids remains uncertain (Non Patent Literature 1, Figure 7.24 A and B, and Non Patent Literature 2), little progress has been made in the identification of genes involved in the biosynthetic pathway.
While glycoalkaloids have toxicity such as a cholinesterase inhibitory activity and a membrane disrupting effect, they are also known to have medical actions such as an anti-cancer activity, a liver-protection effect, an antispasmodic effect, an immune system-promoting effect, an antifungal effect, an antiprotozoal effect, and a molluscicidal activity (Non Patent Literature 1). It is also reported that esculeoside A, which is a metabolic product of glycoalkaloid in tomatoes, exhibits an anti-arteriosclerotic action (Non Patent Literature 3). However, since the biosynthetic pathway remains unknown, little progress has been made in research and development of inhibition or efficient production of metabolic products.
Recently, there are some reports on genes involved in the transglycosylation process after the production of aglycone (Non Patent Literatures 4 to 6). However, although Non Patent Literature 4 reports the gene of UDP-galactosyltransferase, which mediates the conversion of solanidine, which is an aglycone, to γ-solanine, and reports a strain in which the gene is suppressed, chaconine is not suppressed at all (Non Patent Literature 4, Figure 2). Non Patent Literature 4 reports the gene of UDP-glucosyltransferase, which mediates the conversion of solanidine to γ-chaconine, and reports a strain in which the gene is suppressed, either of chaconine or solanine is hardly suppressed (Non Patent Literature 5, Figure 5). Non Patent Literature 6 reports the gene of rhamnosyltransferase, which mediates the conversion of β-chaconine to α-chaconine, and β-solanine to α-solanine, showing that although the α form is decreased, the β and γ forms are increased. As shown above, it is understood that although the molecular species of glycoalkaloids can be changed also by inhibition of the transglycosylation process, control of the total amount of glycoalkaloids is extremely difficult.
There is a report of an attempt to reduce glycoalkaloids through overexpression of genes involved in the biosynthesis of plant sterols and plant hormones (Non Patent Literature 7). However, the amount of glycoalkaloids has only been reduced by about half at most (Non Patent Literature 7, Figure 5).